M.Z. Ma

Development of a new β Ti alloy with low modulus and favorable plasticity for implant material.

One of the most important development directions of the Ti and its alloys is the applications in medical field. Development of new Ti alloys with low elastic modulus and/or favorable biocompatibility plays an important role for promoting its application in medical field. In this work, a new β Ti alloy (Ti-31Nb-6Zr-5Mo, wt.%) was designed for implant material using d-electron alloy design method. Microstructure and tensile properties of the designed alloy after hot rolling (HR) and solution followed by aging treatments (SA) were investigated. Results show that the designed alloy is composed of single β phase. However, microstructural analysis shows that the β phase in the designed alloy separates into Nb-rich and Nb-poor phase regions. The Nb-rich regions in HR specimen are typical elongated fiber texture, but are equiaxed particles with several micrometers in SA specimen. Tensile results show that the designed alloy has low Youngs modulus of 44 GPa for HR specimen and 48 GPa for SA specimen which are very close to the extreme of Youngs modulus of bulk titanium alloys. At the same time, the designed alloy has favorable plasticity in term of elongation of 26.7% for HR specimen and 20.6% for SA specimen, and appropriate tensile strength over 700 MPa. In short, the designed alloy has low elastic modulus close to that of bone and favorable plasticity and strength which can be a potential candidate for hard tissue replacements.

Dynamic precipitation of Al–Zn alloy during rolling and accumulative roll bonding

In this study, cold rolling was performed on a binary Al–20 wt%Zn alloy and dynamic precipitation identified for the first time in Al alloys under cold rolling. Zn clusters formed after application of 0.6 strain, and the Zn phase precipitated upon further increasing strain. Both grain refinement and rolling-induced defects are considered to promote Zn precipitation. The hardness of Al–Zn alloy initially increased with strain up to a strain of 2.9 and then decreased with increasing rolling strain. Dynamic precipitation greatly affects the strengthening mechanism of the rolled Al–Zn alloy under various strains.

Tribological properties of plasma-nitrided AISI 4340 steel in vacuum

Tribological properties of nitrided AISI 4340 steel are evaluated using ball-on-disk tribometer in vacuum. Optical microscopy and Vickers hardness tester are used to analyse compound layer thickness and case depth. X-ray diffraction is used to observe the presence of different constituents of nitrided surface before and after the wear tests. The wear mechanisms are investigated by microscopic observations of the worn surface using scanning electron microscopy. At 200 rpm, mild adhesive wear forms the main wear mechanism under the lower load. With increasing the normal load, the dominant wear mechanism changes to mild plastic deformation and abrasive wear. At 600 rpm, the wear mechanisms vary from the mild plastic deformation to severe plastic deformation, abrasive wear and even delamination with increasing the normal load.

Preparation of high-strength Al-Mg-Si-Cu-Fe alloy via heat treatment and rolling

An Al-Mg-Si-Cu-Fe alloy was solid-solution treated at 560°C for 3 h and then cooled by water quenching or furnace cooling. The alloy samples which underwent cooling by these two methods were rolled at different temperatures. The microstructure and mechanical properties of the rolled alloys were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, and tensile testing. For the water-quenched alloys, the peak tensile strength and elongation occurred at a rolling temperature of 180°C. For the furnace-cooled alloys, the tensile strength decreased initially, until the rolling temperature of 420°C, and then increased; the elongation increased consistently with increasing rolling temperature. The effects of grain boundary hardening and dislocation hardening on the mechanical properties of these rolled alloys decreased with increases in rolling temperature. The mechanical properties of the 180°C rolling water-quenched alloy were also improved by the presence of β″ phase. Above 420°C, the effect of solid-solution hardening on the mechanical properties of the rolled alloys increased with increases in rolling temperature.

High-cycle fatigue behaviour and mechanism analysis of a forged TiZr-based alloy

Fatigue resistance, particularly the endurance limit, is an important design consideration in engineering applications for TiZr-based alloys. The investigated Ti–20Zr–6Al–4 V (wt-%) alloy exhibited a high fatigue endurance limit of 775 MPa. Results showed that severe local stress concentration due to extensive dislocation pile-up at α/β interfaces was responsible for the crack initiation. A transition from a tensile mode to a shear mode crack was observed during crack propagation. Many striations as well as some micro-cracks which can improve the resistance to crack propagation exist in the stable crack-propagation region. A localised deviation between the crack-growth direction was also found, and this outcome combined with micro-cracks and tear ridge may be attributed to varied crystallographic orientations between different phases.

Formation and evolution of shear bands in Zr40Ti60 alloy impacted by Hopkinson bar

Abstract Shear band formation and evolution in Zr40Ti60 alloy impacted by split Hopkinson pressure bar at different strain rates were investigated. The critical compression strain for the initiation of the shear band was about 15%, and almost no changes were observed with increasing strain rate. The microstructure of the shear band with a width of 13 μm was characterised with elongated and broken grains. Moreover, nanosized dynamic recrystallisation grains (50–100 nm) were observed at the centre of the shear band with widths of 41 and 68 μm. In addition, the microhardness at the centre of the shear band decreased from 560 to 410 HV when the shear band width increased from 13 to 68 μm.

Tribological Behavior and Wear Mechanism of TZ20 Titanium Alloy After Various Treatments

Abrasion is one of the most common failure forms of metals and alloys. The study of the friction and wear behavior of metals and alloys is greatly beneficial to improve their long life and safe service. The friction coefficient, weight loss, specific wear rate, and wear mechanism of Ti-20Zr-6.5Al-4V (TZ20) alloy after various treatments were investigated using a pin–disk-type wear apparatus at various normal loads. The wear results revealed that at a normal load, the friction coefficient steadily decreased, whereas the weight loss increased. By contrast, different specimens displayed distinct variation trends of specific wear rate with respect to the normal load. Further investigations indicated that the effects of hardness and toughness on the wear properties were markedly influenced by the normal load. The main characteristic of the abrasion surface gradually changed from grooves to plastic deformation as the normal load increased. The findings promote the practical application of TiZrAlV series alloys and expand wear theory.

Formation and evolution of shear bands in cold rolled Zr40Ti60 alloy

Abstract The microstructure evolution and formation of shear bands in Zr40Ti60 alloy during different cold rolling processes were studied in this article. Results indicate that the width of the shear band formed in the sample increases rapidly with the increasing of the thickness reduction of the sample, and the temperature rise caused by plastic deformation played an important role in the evolution process of the shear band. The microstructure within the shear bands much finer than that in the surrounding matrix has been confirmed, and nanosized subgrains were observed at the centre of the shear band with a width of 15 μm. Hardness tests showed that the microhardness within the shear band with a width of 15 μm (537 HV) was markedly higher than that in the surrounding matrix (410 HV).